28. Heat transfer in fluids

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Heat transfer in fluids

Another way that heat energy can be transferred is when you heat a fluid (a liquid or a gas) to create a convection current. For example, when you put your hand above a hot radiator you'll feel some air rising - this is a convection current.

When a fluid is heated, the hotter fluid particles gain more kinetic energy, move around faster and so take up more space between them. As a result, the hotter fluid becomes less dense (there are fewer particles in the same volume) and so they will rise. At the same time, any cooler fluid will fall back down to take the place of the hotter fluid.

Some examples of convection currents include:

  • The movement of molten rock in the Earth's mantle. The hot rock heated by the Earth's core becomes less dense and rises (slowly!). Cooler rock falls to take its place.
  • Heating some water with a coloured dye (potassium permanganate crystals)
  • Sea breezes happen because the sun warms up the land faster than it warms up the sea. Warmer air rises up from the land pulling in cooler air from the sea, creating a 'sea breeze'.

Important note: Remember that the fluid particles do NOT expand when they get warmer. It's the SPACE between the particles that gets larger :).

GCSE Physics Keywords: Convection currents, fluids, gas, liquid, less dense, rise

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